When transmitting a digital signal via a transmission line, a part of signal energy may be reflected at a timing when a signal level changes in a receiving side, and waveform distortion such as overshoot or undershoot, that is, ringing may occur in the signal. Various techniques for suppressing waveform distortion have been proposed. For example, Patent Literature 1 discloses a technique for suppressing ringing by matching impedances only for a certain period in which communication is not affected by transition of voltage level between a low level and a high level in the transmission line.